php is active by default.
to be clearer, php is just an extension in your Apache. There is no start button in your control panel, because it's not needed.
If you start your Apache, he will be able to route your php files through the php interpreter.
Just copy your php files into your htdocs folder (or a subfolder of it), open your browser and type in the url.
e.g. if you have a test.php file is in your Folder c:/xampp/htdocs/, the url will be http://localhost/test.php